Process and apparatus for depositing metals on a support by thermal evaporation in avacuum



July 6, 1948. p ALEXANDER 2,444,76$

PROCESS AND APPARATUS FOR DEPOSIIING METALS ON A SUPPORT BY THERMALEVAPORATION IN A VACUUM Filed 001 7. 1946 v v i 26 j 26 A 2 25 I! I! 26a""5 V, I ill lll Inventor Patented July 6, 1948 UNIT-EDI STAT-Es PATENToFFicE ING'ME'frA Js A SUPPORTfBY THERMAL EVAPORATIONIN A VACUUM PaulAlexandenBe'rkhamsted, Eng land Ali-mammo ram 7, 1946,"Ser ial' No.7013158 ratus for depositing metals on asupport by thermal evaporationin a vacuum.

. The metal most commonly evaporated is aluminium and the heaterrequired for its evaporationis one of the .rafractory metals, tungstenbeing: the most suitable.

Aluminium, however; at high temperature alloys with or dissolvestungsten, and the tungsten heater quickly becomes rotten and breaks ifmolten aluminium remains in contact with it.

. In U. S. A. patent specification No. 2,153,786 there is disclosed 'aprocess of and apparatus for depositing aluminium on a support inavacuum, the heater being made of tungsten, and in which a means avoidingthe rapid disintegratioh of the heater is avoided by feeding aluminiumto the heater in the form of a wire so that an end only of the wire cameintocontact with'the heater, and was evaporated before there was timefor a pool of :molten aluminium to. form onthe bar. By this means thelife of the tungsten heater can ,be very largely increased.

Nevertheless, the tungsten disintegrates in time, and the wastage oftungsten remains the most costly ite'm inthe process.

The principal object of the invention is to reduce the-wastage oftungsten .in heatersincorporated in apparatus of thekinddescribed.

Another object is to avoid dense aluminium vapour in the vicinity of'theheater.

Another object is to. so feed a fine wire of the metal to be evaporatedso as to assure that it is brought into contact, with a tungsten heaterof very small cross-section. e

I Another object isto lead currentto a tungsten heaterin rod orbar formby meanswhich avoid stressing-the heater whenit lengthens or contractson heating and cooling.

To avoid accumulation of vapourin the vicinity of the heater thealuminium wire'must'be so fine, that it is .very flexible, and it hasbeenfound impracticable to feed such a wire, if straight, accurately toa given point.

Guiding means for the wire cannot be brought close to the heater bar, byreason of the high temperature radiation, .aridfalso' because any such'me'a'ris, close to the point of evaporation, becomes covered with agrowing layer of deposited aluminium. A length of wire of 2.5 to 5 cms.must therefore be allowed to descend free of any guiding means, and thislength is liable to be deflected from a straight path by five or moremillimetres.

Apparatus for depositing metal on a support by tinuously 3 Claims. (01.117-107) 2 thermal evaporation-of the metal in a vacuum. comprises aheater bar of-refractory metal, winding means disposedv with itsv axisvertically above theheater and perpendicular thereto. for condrawing awire of the metal to be evaporated from a storage spool and a coaxialformer'on which the wire is coiledintoa contlnuously rotating andvertically descending helix, said former terminating at a, point'distant from the heater, whereby .a free length of helix is disposed inthe vicinity of thehe'ater and the lower end only of the helix comesinto contact' with'the heater bar.

Preferably the descending helix has a diam- 'eter which is larger thanthe width of the heater bar.

By the present invention, a fine aluminiumwire may be :fed to a'heaterrod or bar of tungsten of as small a crosssection as is desired, with no'risli of the wire failing to make contactwith it. In practice a roundrod of tungsten is the most economical form of heater bar, and a rod ofonly two millimetres diameter may be used. The invention,thereforefenables a smallsamount of tungsten in'its most economical'formto be used in metal depositing apparatus of the kind described.

The invention comprises also a process ofrdepositing aluminium on asupport .by thermal evaporation in a vacuum. wherein a tungsten heateris used to evaporate the metal and the metal in the form of a wire iscontinually ad'- vanced to the heater so that the end only of the wiretouches theheater, characterised in that the metal is fed .to the heaterin theform of a freely dependinghelix rotating about its'axis, the rateof descentof the wire in the helix being related to the temperature ofthe heater to effect instantaneous evaporation of the wire as it comesin contact with the heater, sents only a relatively small helix, the'radius of the helix being somewhat greater'than the extent of maximumdeflection of the'free endof the helix laterally of the heater. Onepreferred form of the invention which has been" found successful inpractice in the process of depositing aluminium on a support by thermalevaporation in a vacuum, will be described, by way of-exai'nple, withreference to the accompany' ing drawings, of which- Figure 1 is a viewfrom below of the helix forming device, in which the helix itself isomitted, taken along the line AA of Figure 2.

Figure 2 is a side view of the device including the heater bar, aportion of the helix forming and'the heater p-recrosssection to thespindle 8 is turned 3 against the portion :and. serves to guide .as aformer.

the heater bar and its deflection of even several millimetres ispermissible.

The helix making device is mounted on a plate I fixed to the wall 2 ofthe vacuum chamber, which latter may be constructed as described in theaforesaid United States patent, and com prises a floor 2c in which is apipe 2?) through which the chamber is evacuated. The wire 3 is takenfrom a spool 4 through a guide tube 5, with lead in bell-mouth B, thetubebeing held in a support I fixed to the plate I.

- A spindle 8 is adapted to turn in a bush 9 which passes through theplate I and above the bush a chain wheel I 0 is fixed by a set screw IIto the spindle. The chain wheel ill, with the slowly by a chain t2 froma driving. chain wheel, I3, which may be driven in any convenient way,such as the clockwork shown diagrammatically by the casing I 4, fixed tothe plate I by the bracket Ma. A wire wind- ;ing portion I5 of thespindle 8 is knurled, and the tube 5 is located so that the wire 3issues tangentially to the winding portion l5. A roller I6 heldin an armII, pivoted to the plate I at I8 is urged towards the knurled portion I5of the spindle by a spring I 9, and then presses the wire I5. A guidenib fixed a groove 2i in the spindle the wire towards a lower portion 22of the spindle, which portion functions A support S on which the vapouris. to be deposited is carried in a frame F mounted on the floor 2a.

In operation, the wire is drawn by hand through the tube 5 between theknurled portion I5 and the roller I6, passed below the guide nib 20 andgiven one or two turns round the former portion 22. Then, when thespindle is turned counter-clockwise as seen in Figure l, the wire 3 isdrawn from the spool by the winding portion I5 and continuouslytransferred to the former 22 where it is coiled into a continuouslyrotating and vertically descending helix 23 of about 8 mms. diameterwhich extends beyond the former to the plate I, enters thereby providinga free length of the helix .which reaches the heater only the free endof .the Wire touching the heater.

The helix-forming device is mounted 2!) cans. or more above the point ofevaporation to avoid its becoming thickly coated with the evaporatedmetal. The helix 2 3 tends to descend to the point of evaporation in astraight line, but it may 'be guided by a rod 24, forming an extensionof the spindle to within 2.5 to 5 cms. of the point of evaporation.

The heater bar consists of a thin rod 250i about 2 mms. diameter oftungsten and presents a relatively small cross section to the helixwhich has a diameter of 8 mm., and the invention commounted in the 'bar30, the

-metal to be evaporated from a prises a novel means of supporting suchheater.

To this end the heater is gripped between blocks 26 of copper supportedon posts 25a passing through the floor 2a and insulated therefrom. Theblocks 26 are carried on supports 21 of copper comprising strip portions28 which are thin enough to yield and permit the expansion andcontraction'of the tungsten rod 25. Each support 21 is electricallyconnected to a lead 29 by a bar so fitted to the respective post 2 6a,and is lead being clamped to its post by a nut 31.

' In operation, the rod 25 is heated to a temperaturein its central partof some 2000 0., the wire in the helix 23 is fed at a rate related tothe temperature of the heater so that the end of the I helix melts andis evaporated as soon as it comes into contactwith the rod 25, and thereis no risk of the helix deflecting enough to make it fail to contact therod. j

I claim:

1. Apparatus for depositing a metal on a support by thermal evaporationof the metal in a vacuum, comprising a heater bar of refractory metal,winding means disposed with its axis vertically above the heater andperpendicular thereto for continuously drawing a wire of the storagespool and a coaxial former on which the wire is coiled into acontinuously rotating and vertically descending helix, said formerterminating at a point distant from the heater, whereby a free length ofhelix is disposed in the vicinity of the heater and the lower end onlyof the-helix comes into contact with the heater bar.

2. Apparatus for depositing a metal on a support by thermal evaporationof a metal in a vacuum, comprising a heater bar of refractory metal,winding means disposed with its axis vertically above the heater andperpendicular thereto for continuously drawing a wire of the metal to beevaporated from a storage spool and a coaxial former on which the wireis coiled into a continuously rotating and vertically descending helixof a diameter larger than the width of the heater bar, said. formerterminating at a point distant from the heater, whereby a free length ofhelix is disposed in the vicinity of the heater and the lowerend only ofthe helix comes into contact with the heater bar. I

3. Process of depositing aluminium on a support by thermal evaporationin a vacuum wherein a tungsten heater is used. to evaporate the metaland the metal is in the form of a wire continually advanced to theheater so that the end only of the wire touches the heater to evaporatethe metal, characterised in that the wire is fed to the heater in theform of a freely depending helix rotating about its axis, the rate ofdescent of the wire on thehelix being related to the temperature of theheater to eflect instantaneous evaporation of the wire as it comes incontact with the heater, and the'heater presents only a relatively smallcross section to the helix, the radius of the helix being somewhatgreater than the extent of maximum deflection of the free end ofthe'helix laterally of the heater.

. PAUL ALEXANDER.

